1. Field of Application
The present invention relates to an electronic scanning type of radar apparatus, and in particular to a vehicle-installation FM-CW (frequency modulation, continuous wave) or CW (continuous wave) type of electronic scanning radar apparatus which enables an interference component contained in a received signal to be suppressed.
2. Description of Related Art
Types of radar apparatus for vehicle installation are known which can detect the distance, velocity and direction (i.e., azimuth bearing) of a target object such as a preceding vehicle, for use in collision avoidance and vehicle control. A vehicle equipped with such a radar apparatus, and for which the apparatus operation is being described, will be referred to in the following as the local vehicle. One method of achieving this is to utilize FM-CW radar. This has advantages such as simplicity of the circuit configuration required for signal processing, etc.
The upper and lower timing diagrams of FIG. 1 are timing diagrams for describing the principles of mixing of transmitted and received signals with FM-CW radar. S1 denotes a signal that is transmitted as electromagnetic waves from an antenna by a FM-CW radar apparatus of the local vehicle, and whose frequency varies linearly with time, in alternating intervals of frequency increase and frequency decrease as shown in the upper timing diagram of FIG. 1. When the transmitted signal S1 is reflected from a target object, a corresponding received signal S2 is obtained. The transmitted and received signals S1, S2 are mixed, to obtain the result shown in the lower timing diagram of FIG. 1. As indicated, during each interval in which the signal S2 is being received concurrent with the signal S1 being transmitted, a beat signal component having a frequency (beat frequency) fb is obtained as a result of the signal mixing, where fb is the difference between the respective frequencies of the transmitted signal S1 and received signal S2. The value of the beat frequency varies in proportion to the delay time Δt required for the transmitted signal transmitted waves to travel from the radar apparatus of the local vehicle to a target object and return to that radar apparatus. Hence, the distance to the target object can be calculated based on fb.
One method of measuring the direction of a target object is to utilize electronic scanning. This method enables scanning in all directions to be performed in a short time. Radar waves (transmitted electromagnetic waves) reflected back from a target object are received by a plurality of antenna elements of an array antenna, which are disposed in a specific position arrangement. Differences arise between the respective times at which the reflected waves are received by the antenna elements, is resulting in corresponding phase differences between the received signals from the antenna elements. The antenna signals are down-converted in frequency to obtain respective down-converted beat signals of respective channels (i.e., corresponding to respective antenna elements). Each of the beat signals is periodically sampled by an A/D converter to be converted to corresponding channel data, i.e., as channels signals. Phase differences between the channel signals (resulting from phase differences between the corresponding antenna element signals) can be used to detect the direction of the target object. One form of this method is known as DBF (digital beam-forming). With DBF, after A/D (analog to digital) conversion of the channel signals, the direction of a target object is obtained based on correlating the phase relationships of the channel signals with a mode vector which expresses the direction of arrival of received radar waves reflected from a target object. This is described for example in “Adaptive Signal Processing with Array Antennas”, published in Japan by the Science Technology Publishing Co, referred to in the following as reference document 1.
With that electronic scanning method, data (sample values) are acquired concurrently for each of the channels by periodic sampling (A/D conversion operations). However this makes it necessary to utilize separate A/D converters for each of the channels, so that the apparatus becomes complex and expensive.
As illustrated in FIG. 5 of the drawings (described in detail hereinafter) it is known that a channel selector switch 7 can be used to perform switched selection of the respective signals from the plurality of antenna elements, i.e., for TDM (time-division multiplexing) reception of the antenna signals. This is described for example in reference document 1.
In that case, designating the number of an antenna element as k (i.e., as counted in the switching sequence from a 1st antenna element) there will be a switching delay τ between the signals of adjacent channels, i.e., with A/D sampling synchronized with the signal switching, there will be a switching delay τ between sampling respective signals of adjacent channels. Hence the timing of acquiring the antenna signal of the k-th antenna element will be delayed by an amount kτ with respect to the 1-st antenna element. If the maximum value of the switching delay τ[k] is negligible by comparison with the period of the beat signal (i.e., τ[k]<<1/fb) then processing can be performed on the assumption that antenna signals are received concurrently from the respective antenna elements. However in practice, from reasons of cost, etc., it is generally necessary to use circuitry for the channels selector switch 7 that is only capable of relatively low-speed antenna signal switching, so that the channel selector switch 7 can only be operated at a relatively low drive frequency. Hence in general, the delay time τ[k] cannot be ignored Specifically, if the phase deviations between channel signals due to this switching delay are large, then the accuracy of detecting the direction of a target object will become lowered. Hence it is necessary to apply phase compensation to the channel signals by a phase amount Δφ[k] which is expressed as follows:Δφ[k]=2·π·fB·τ[k]  (1)
Applying such phase compensation processing enables accurate direction detection to be achieved, even when multiplexing of antenna signals is utilized. However another problem exists, which will be described referring first to FIG. 2. Here, vehicles which are each equipped with an FM-CW radar apparatus are travelling in opposite directions to one another. Hence, a local vehicle 50 which is transmitting electromagnetic waves (radar waves) Tx receives radar waves Rx2 that are transmitted by a vehicle 52 which is running in an opposing traffic lane. The local vehicle 50 also acquires receives reflected radar waves Rx1, resulting from reflection of the radar waves Tx by a target object (a preceding vehicle 51).
Since the radar waves Rx2 are directly received by the local vehicle 50 from the transmitting source, they may be received at a substantially higher level of signal power than the reflected waves Rx1. Thus, accurate phase information cannot be obtained for the desired received signal components (resulting from the reflected radar waves Rx1), so that the direction of the target object cannot be estimated.
This problem can be overcome by suppressing the interference components. One method which has been proposed for achieving this utilizes a filter for suppressing frequency components corresponding to received electromagnetic waves which arrive from a specific direction. Such a method is described for example in a technical paper “Adaptive Mainbeam Jamming Suppression for Multi-Function Radars”, by T. J. Nohara et al, referred to in the following as reference document 2. However with a time-division multiplexing type of radar apparatus, it may not be possible to accurately determine the arrival direction of the interference waves. The reasons for this will be described in the following.
When the interference waves Rx2 are generated from an FM-CW type of radar apparatus they vary linearly in frequency, as do the transmitted waves Tx from the local vehicle, as illustrated in FIG. 3A. As illustrated in FIG. 3B, when an antenna signal which results from receiving Rx1 and Rx2 is supplied to the signal mixer 10, the resultant down-converted signal (beat signal) S3 produced from the mixer will contain an interference component Ix resulting from the interference waves Rx2. During each interval in which the frequency of the interference component Ix is above the Nyquist frequency of the A/D converter, aliasing components will appear in the sampled channel signals (i.e., in the digital data which should express the channel signals). The aliasing components vary in frequency as illustrated in FIG. 3C.
In such a case, it may not be possible to use phase relationships between channel signals for detecting the direction of an interference source, for the purpose of suppressing an interference signal transmitted from that source, or to perform phase compensation for time delays between the signals of respective channels. This problem due to aliasing arises both for a FM-CW radar apparatus which utilizes concurrent reception (with separate A/D converters for the respective channels) and for a FM-CW radar apparatus which utilizes TDM of antenna signals (with a single A/D converter).
If the slopes of the respective FM-CW modulation characteristics (the frequency/time variation characteristics of Rx1, Rx2) are close to being parallel, then the frequency of the resultant interference component Ix will be correspondingly low. However if the slopes are substantially different, as in FIGS. 3A, 3B and 3C, then the frequency of the interference component Ix will exceed the Nyquist frequency for A/D conversion, causing aliasing to occur. In that case, the phase relationships between the channel signals prior to A/D conversion (i.e., prior to aliasing) will not correspond to those of the sampled channel signals. Thus, channel data thus obtained cannot be processed to obtain the direction of an interference source, so that the interference signal cannot be suppressed.
The reasons for this will be described referring to FIGS. 4A and 4B, showing waveforms of channel signals before and after sampling, when a large-amplitude interference component of each channel signal (before sampling) has a frequency that exceeds the Nyquist limit for aliasing at A/D conversion. Signals of three channels (channel 1, channel 2, channel 3) are shown, with channel 2 corresponding to the central element of an array antenna. FIG. 4A illustrates the case in which the channel signals are sampled concurrently at data timings 0, 1, 2, etc. Each full-line curve AS shows the waveform (i.e., locus of successive data values) of a sampled channel signal, while each broken-line curve BS shows the waveform of a channel signal before sampling. This example assumes that interference waves are received from a transmitting source located directly ahead of the local vehicle (the 0° direction) so that all of the before-sampling channel signals are in phase with the channel 2 signal.
It will be understood that in the case of FIG. 4A, if the direction of the received interference waves is other than the 0° direction, so that phase differences arise between the before-sampling channel signals BS at each of the time points 0, 1, 2, etc., these will not correspond to resultant phase differences between the sampled channel signals AS, due to the aliasing, so that the direction of an interference source cannot be estimated based on the sampled channel signals AS.
FIG. 4B illustrates the above for the case of TDM reception, causing the above-described switching delays τk. In FIG. 4B, the “x” symbols indicate the sampling timings of the TDM channel signals. In this case, although interference waves are again assumed to be received along the 0° direction, and although the switching delay time τ is predetermined, it is not possible to apply compensation for the switching delays to the after-sampling channel signals AS, due to the occurrence of aliasing. Due to this, and to the reasons described for FIG. 4A above, the direction of an interference source cannot be estimated, so that interference components in the after-sampling channel signals AS cannot be suppressed based on that direction.
For the above reasons, a technique fox suppressing such interference components in the channel signals has been proposed in U.S. patent publication No. 2008-0036645, designated as reference document 3 in the following, whereby when a time-axis interval of each of the sampled channel signals (A/D converted beat signals) has been converted to a corresponding channel data set comprising a fixed number of data values, a plurality of short data segments are extracted from each channel data set. The short data segments respectively correspond to sequential data timings, and the duration of each short data segment is sufficiently short that the interference component frequency does not change significantly within each short data segment. Respective frequency spectra of these short data segments are then obtained. A plurality of candidate values of frequency of the interference signal are then estimated for each of the short data segments, based on these frequency spectra, and DBF (digital beam forming) is then applied to obtain the most suitable candidate value, which is used to estimate the most probable direction of arrival of the interference waves. That information is then used in suppressing the interference components in the channel signals.
However with the method of reference document 3, it is necessary to derive a plurality of candidate values of pre-aliasing frequency of the interference component, and to perform DBF processing based on each of these candidate values, In particular, if it is necessary to use a relatively high value of cut-off frequency for low-pass filtering that is applied prior to A/D conversion, then the number of candidate values of pre-aliasing frequency will be large. Hence, the required number of calculation operations becomes excessive, and since a radar apparatus which performs real-time processing must execute that processing at high speed, it becomes necessary to use a plurality of high-performance processors with respective calculation programs and calculation units.